The present invention relates to an iced-beverage making machine, such as a water-ice making machine. Generally, these machines involve the presence of a tank containing the product to be dispensed and in which a mixing propeller and an evaporator for a beverage refrigerating circuit are present. A problem felt in these machines is to avoid complete freezing of the beverage, keeping the beverage itself close to a preestablished optimal density value. The desired density value is included in a rather narrow range: an excessive density increase produces quick stopping of the propeller, or at all events overload of the motor, whereas a too low density results in a beverage qualitatively unacceptable by a consumer.
Unfortunately, in iced-beverage making machines the cooling system is not controllable based on the beverage temperature. In fact, iced beverages are a mixture of water, sugar and other additives that are kept in an equilibrium state between the liquid and solid phases and their density is a function of the relative percentages of the two phases. Since the physical phenomenon of the change of state from liquid to solid is a phenomenon taking place at a constant temperature, this temperature will not vary during the change of state, which will make it impossible to control the equilibrium state through the temperature. In the known art, several different systems for detecting parameters that are indicative of the true density of the product were proposed for use in the refrigeration control. Generally they are based on detection of operating magnitudes supplying an indication of the torque offered by the iced beverage on movement of the mixer. For instance, measurement of the values of the electric current absorbed by the motor operating the mixer was proposed, using then such values as indices of the resisting torque on the mixer propeller. Unfortunately however, in the case of small electric motors using reduction gears for increase of the delivered torque, the absorbed current does not supply a reliable adjustment parameter.
Thus, mechanical systems for torque detection were proposed that can presently be considered, in their different embodiments, as the universally adopted systems for refrigeration control. Generally, these mechanical systems involve a certain freedom degree in the electric motor mounting that for instance can rotate around its output shaft against a resisting force (a spring or the weight of the power unit suitably eccentrically arranged, for example). To enable calibration of the intervention density, a preload spring is provided that counteracts the movement of the power unit due to the resisting torque on the propeller.
On increasing of the resisting torque on the propeller, the power unit rotates through an increasingly wider angle until a microswitch controlling turning off of the refrigerating system is operated. Once the refrigerating system has been turned off, density will begin lowering by effect of the iced beverage melting. Consequently the resisting torque will decrease causing return of the power unit to its starting position and shutting off of the microswitch which will give rise, as a result, to restarting of the refrigerating system.
Even if in more sophisticated systems the microswitch was replaced by a different sensor detecting rotation of the power unit, such as an optical photodiode system, all devices of the known art suffer from the disadvantage of an excessive mechanical complexity, and from the requirement of providing appropriate spaces for movement of the whole power unit under the action of the resisting torque. In addition, reliability and constancy of intervention rely on the preload spring counteracting the movement of the power unit, which spring needs an accurate starting calibration and periodical adjustments.
It is a general aim of the present invention to obviate the above mentioned drawbacks by providing an iced-beverage making machine which is equipped with a precise, simple and reliable system for detecting the density of the product in order to control of the beverage refrigeration.
In view of the above aim, in accordance with the invention, an iced-beverage making machine has been devised which comprises a beverage-containing tank, a mixer to stir the beverage in the tank which is connected by a magnetic joint to a movement reduction motor and a refrigerating circuit to cool the beverage in the tank, the magnetic joint comprising a driving portion which is provided with a crown of first magnets and is connected to the reduction motor, and a driven portion which is provided with a crown of second magnets and is connected to the mixer and magnetically driven in rotation by the driving portion, characterised in that close to the two magnet crowns there is the presence of means for detecting the angular phase displacement of the two crowns relative to the angular position of maximum attraction between the magnets, the detecting means sending a disabling signal to the refrigerating circuit when the detected angular phase displacement exceeds a preestablished value.